A 5G physical layer aims to provide high performance data rate and reduced cost and power consumption. Massive MIMO, high frequency bands and dense deployment of small cells are expected to be main techniques to meet high capacity requirements of 5G. Besides, system scalability, flexibility of keeping UL and DL capacity, and optimized usage of unpaired spectrum are very important for 5G concept. Moreover, in addition to traditional UE-eNB access, more and more new various communication links are emerging such as self-backhauling by means of eNB-eNB communication and device-to-device communication without infrastructure, etc.
Based on these requirements of 5G, it is envisioned that the TDD mode has significant advantages for future 5G solutions, considering its cost-effectiveness as well as the possibility of exploiting large unpaired frequency bands. Use of the TDD model also allows exploiting channel reciprocity between UL and DL for reducing the feedback overhead, which is very beneficial for massive MIMO techniques requiring extensive channel state information.
However, in the current LTE/LTE-Advanced systems from Release 8 to Release 12, FDD operation is a dominating duplex mode compared to the TDD mode although both operations are supported. The TDD mode has been greatly harmonized to the FDD mode and the degree of TDD-specific optimization has been minimized. Specially, the design of subframe structure is optimized to FDD rather than TDD. With the conventional FDD-optimized subframe structure in use, in the TDD mode, the duration of any hand-shaking procedures between UE and eNB such as the initial data scheduling and the round trip time (RTT) of HARQ process is extended and highly dependent on the UL/DL ratio. This cannot achieve the ambitious RTT requirement (1 ms) of future 5G technology.